The present invention relates to laser-markable plastics comprising non-lustrous pigments based on phyllosilicates which have an irregular surface.
The marking of products is becoming increasingly important in virtually all sectors of industry. Thus, for example, production dates, use-by dates, bar codes, company logos, serial numbers, etc., must frequently be applied. At present, these marks are predominantly made using conventional techniques such as printing, embossing, stamping and labelling. However, the importance of non-contact, high-speed and flexible marking using lasers is increasing, especially in the case of plastics. This technique makes it possible to apply graphic inscriptions, for example bar codes, at high speed even on a non-planar surface. Since the inscription is in the plastic article itself, it is durable and abrasion-resistant.
Many plastics, for example polyolefins and polystyrenes, have hitherto proved to be very difficult or even impossible to mark by means of lasers. A CO.sub.2 laser which emits light in the infra red region at 10.6 .mu.m produces only a weak, barely legible mark in the case of polyolefins and polystyrenes, even at very high output levels. In the case of elastomers which are polyurethane and polyether esters, Nd-YAG lasers produce no interaction, while CO.sub.2 lasers produce an engraving. The plastic must not fully reflect or transmit the laser light, since no interaction then occurs. Neither, however, can there be strong absorption, since in this case the plastic evaporate and only an engraving remains. The absorption of the laser beams and therefore the interaction with the matter depends on the chemical structure of the plastic and on the laser wavelength used. In many cases it is necessary to add appropriate additives, for example absorbers, in order to render plastics laser-inscribable. The laser marking of plastics is increasingly being carried out using Nd-YAG lasers in addition to CO.sub.2 Lasers. The YAG lasers usually used emit a pulsed energy beam having a characteristic wavelength of 1064 nm or 532 nm. The absorber material must exhibit pronounced absorption at the laser wavelength used in order to exhibit an adequate reaction during the rapid inscription operations.
DE-A 29 36 926 (corresponding to U.S. Pat. No. 5,053,440) discloses that the inscription of a polymeric material by means of laser light can be achieved by admixing the plastic with a filler, such as carbon black or graphite, which discolors on exposure to energy radiation.
EP 0 400 305 A2 (corresponding to U.S. Pat. No. 5,350,792) describes highly polymeric materials which can be inscribed with laser light and which comprise copper(II) hydroxide phosphate or molybdenum(VI) oxide as discoloring additive.
A polymer molding composition based on an organic thermoplastic polymer and comprising a black pigment, which composition can be provided with characters by exposure to laser radiation, is known from EP 0 522 370 A1.
The article "Pearl Lustre Pigments--Characteristics and Functional Effects" in Specialty Chemicals, May 1982, Vol. 2, No. 2 discloses the use of pearl lustre pigments for laser marking. Pearl lustre pigments, however, have the disadvantage that they induce very marked changes in the color properties of the plastic, which is often unwanted.
Further, the fillers known from the prior art have the disadvantage either that they durably color the plastic to be inscribed, as a result of which the laser inscription, which is usually a dark script on a paler background, is then no longer sufficiently high in contrast, or the marking is very weak and only becomes readily visible when high quantities of the additive are employed.
The filler or the successful absorber should therefore possess a very pale, neutral intrinsic color or the properties of the precolored plastic to be marked, or should need to be employed only in very small quantities.